Several different types of displays are commonly used with commercially available personal computers. Such displays can be categorized as either monochrome displays or color displays. Some monochrome displays can display shades of gray.
A display is generally connected to a personal computer through a display adapter. There are many different types of display adapters commercially available; however, most of the commercially available display adapters operate in accordance with one of the defacto standards that exist in this area. Among the defacto standards are standards termed Color Graphics Adapter (generally referred to as CGA), the Enhanced Graphics Adapter (generally referred to as EGA), and the Video Graphics Array (generally referred to as VGA). CGA, VGA, and EGA adapters are widely available from a variety of sources. Each of these adapters has a character mode.
When a video adapter is operating in character mode, there are color control bits associated with each data byte. For example, when a CGA is operating in character mode, each data byte has eight color control bits associated therewith. Four of the color control bits control the background color and four of the color control bits control the foreground color. The eight control bits give sixteen possible foreground colors and sixteen possible background colors for the associated character. FIG. 1A shows a CGA 10A driving a color display 11A. In a standard manner (not explicitly shown) the CGA 10A generates (a) a four bit background color control signal designated BGC (b) a four bit foreground color control signal designated FGC and (c) a one bit background or foreground select signal designated BG/FG. The signals BGC, FGC, and BG/FG are provided to a selector 10As which in turn generates a four bit signal 13A which together with timing signals 12A drive the display 11A. As the CRT beam in display 11A scans the various bit positions on the face of the display, the signals 13A indicate the color that should be displayed at each position. Timing signal 12A insures that the signals on line 13A are in synchronization with the scanning of the appropriate bit positions.
Many widely available publications explain the operation of the color control bits in a conventional CGA adapter and the operation of the system shown in FIG. 1A will not be explained further herein. For more information see a book entitled Inside the IBM PC by Peter Norton which was published by Prentice Hall, 1986 or see a publication entitled "Options and Adapters Technical Reference" publication S229-9612-00 part number 6322509, commercially available from the IBM Corporation.
It is well known that a Color Graphic Adapter, that is, a CGA adapter, operating in character mode can be used to drive a monochrome display. This can be done in a number of different ways. FIG. 1B illustrates one prior art technique for driving a monochrome display 11B from a CGA adapter 10B Monochrome display 11B is driven by a one bit on-off signal 14B which indicates if each pixel on the face of the display should be "on" or "off". In normal operation of a monochrome display the "on" and "off" duty cycles of signal 14B are fixed. CGA 10B generates a four bit color control signals 13B that is identical to signal 13A in FIG. 1A. With the system shown in FIG. 1B, a mapping circuit 15B translates each of the four bit signals on line 13B into a different duty cycle for the signals on line 14B. Thus one color combination on line 13A would generate a long "on" signal on line 14B, whereas a different color combination on line 13B would generate a short signal "on" signal on line 14B. Changing the duty cycle of the signal on line 14B has the effect of producing gray scale signals on display 11B
FIG. 1C shows an example of a prior art system where a CGA drives a Monochrome LCD Display. In the prior art system shown in FIG. 1C, the eight bits of the BGC and FGC signals are mapped into a single bit foreground color signal FG and a background color signal BG. Circuit 10Cs then selects either signal FG or BG in response to signal BG/FG. A single bit signal then goes to LCD display 11C. This type of circuit is, for example, used in a personal computer sold by IBM Corporation under the trademark "PC CONVERTIBLE". For an explanation of how signals are mapped from color to monochrome in the IBM PC Convertible, see a publication commercially available from the IBM Corporation entitled PC Convertible, Technical Reference publication number SA 23-1047, page 2-47 et seq.
As indicated above, in systems where a CGA adapter is used to drive a monochrome display, the sixteen possible foreground colors and the sixteen possible background colors generated by the CGA must be mapped into foreground and background signals available with the monochrome display. For example, in the simplest situation the sixteen foreground and the sixteen background color possibilities from the CGA must be mapped into four "on" and "off" foreground-background possibilities available on a simple monochrome display. If the monochrome display can show different intensities or shades of gray, the number of possibilities on the display may be larger than four; however, there still must be a mapping of color into intensity or gray scale signals.
In the prior art, the mapping from the color signals generated by a video adapter into the signals adapted to drive a monochrome display is done in a manner that is fixed by the design of the machine. Such systems are not very flexible and they do not take into account different user preferences. Furthermore, in order to map all possible foreground-background combinations into a wide variety of gray scale combinations, a substantial amount of logic must be used, hence, in most prior art systems, all possible color combinations are mapped into only a few gray scale values.